site.baseurl ignored when linking to assets

[amw]

This is related to #125 which was already fixed, but the issue creeped up again with the 2.0 changes.

Example Jekyll config:

baseurl: "/my-sub-website"

Now when you use this in your _head.html:

  {% stylesheet "main" %}

You will get:

<link type="text/css" rel="stylesheet" href="/assets/main.css">

Which will produce 404 error, because the asset is served at http://127.0.0.1:4000/my-sub-website/assets/main.css.